Within the diverse fields of laboratory research, cryogenic vessels filled with liquid nitrogen are employed in the storage of biological samples until such time as the laboratory technologist is ready to process or further diagnose the samples. These vessels typically are comprised of an inner enclosure, surrounded by an evacuated outer enclosure, and a cover. The vessel structure minimizes the evaporation of the liquid nitrogen into the surrounding room. Because the covers are not perfectly sealed, leakage still exists, and the liquid nitrogen has to be periodically replenished. There exists the distinct need to know the level of the liquid in the vessel at all times so as to refill the vessel on a timely basis so as to avoid the biological sample from being exposed. It is also desirable to avoid wasting liquid nitrogen as the specimen is transferred to laboratory storage vessels.
A variety of level sensing apparatuses have been suggested in the prior art to appropriately monitor the level of a cryogenic liquid in a vessel so as to provide the user with an indication that the vessel is low or empty. One approach taken in the prior art is through the use of temperature based systems, implying resistant temperature devices and thermistors to detect the temperature differential of the liquid and gas. However, such contact-type sensor devices have two very basic problems. First, these type devices are limited to indicating a particular level and are thus not capable of monitoring the various levels that may be present within the vessel. Additionally, these type devices, due to their fragile nature, are subject to thermal shock. Further, such devices are usually based upon a difference in the resistivity of the sensor. The temperature difference from liquid to gas is so small that sometimes it does not produce a sufficient shift in the sensor resistivity to record an accurate change.
In the prior art there has also been suggested the use of ultrasonic-type sensors, in a contact and non-contact mode. U.S. Pat. No. 3,266,311 is an example of a cryogenic level indicator of the basically go/no-go type, which merely detects the presence or absence of the liquid nitrogen at a specific ,point in the depth of the storage vessel. U.S. Pat. No. 3,170,094 also discloses a go/no-go type device.
There also has been suggested in the prior art the use of multiple sensors to accomplish the continuous analog indication of depth of the liquid. An example of such prior art is illustrated by U.S. Pat. Nos. 2,960,678 and 4,400,936. However, these systems rely on multiple sensors to provide this output reading which, of course, results in additional costs to the system, and in cryogenic applications, would mean significant loss of the storage medium.
Another problem encountered with the liquid level detection system of the prior art is that of system calibration. U.S. Pat. Nos. 3,184,969; 4,210,969; 4,221,004; and 4,470,299 require additional costly hardware components which would be medium wasting in a liquid nitrogen environment and electric circuitry is needed to provide a suitable means of calibration of the system to accommodate for changes in internal vessel pressure and temperature. Other prior art systems failed to easily alert the user to various systems fault conditions. In addition to providing the level indication, very little is done with regard to recognizing such conditions as a fracture of vessel, unstable levels and low level.
Applicants have invented an improved cryogenic level detecting apparatus. An apparatus made in accordance with the present invention incorporates a non-contact single ultrasonic transducer which is strategically positioned in insulation within the vessel which provides a consistent noise-free measurement. The apparatus of the present invention also provides an analog indication of the depth of the liquid. Further, the apparatus according to the present invention, has done away with the need for system recalibration and provides various alerts for numerous fault conditions.